1. Field of Invention
This invention is directed to the field of image capture systems. More particularly, this invention is directed to a solid-state image capture system that includes a holographic polymer dispersed liquid crystal color separation element that can be operated in either a light reflective mode or a transmissive mode to provide fill-color images.
2. Description of Related Art
A new generation of full-color image capture systems based on using solid-state image sensors has enabled digital imaging to penetrate the consumer marketplace. These new systems include both digital cameras for the capture of still images and small video cameras for moving images. Most of the systems in existence today utilize charge coupled devices (CCD) image sensors arrays; however, there are currently many large initiatives for the development of complementary metal oxide semiconductor (CMOS) sensor arrays, and some CMOS-based digital cameras have entered the market. The consumer market for image capture systems is already large and is rapidly growing. This is partially fueled by the Internet and World Wide Web, but is also strongly driven by the allure of digital photography and the availability of low-cost, high-quality color printers capable of rendering photographic quality images. The market for digital camera systems is expected to reach several billion dollars within the next few years and is, of course, highly competitive and cost sensitive.
The principal performance and cost factor for these digital image capture systems is the image sensor itself. In order to minimize the cost of these new color image capture systems and to provide a form factor (i.e., physical size or volume) comparable to film-based cameras, most systems utilize a single image sensor. Color separation is accomplished by a spatially patterned array of red, green and blue (RGB) color absorption filters overlayed upon the image sensor array. The technical challenge for this approach is to utilize the fill spatial resolving capacity of the image sensor while still providing effective color separation. Some method must be incorporated to estimate the "true" luminance signal at each sensor location in the filtered image sensor array. Then, attempts must be made to estimate the color or chromaticity at each sensor location. For example, a digital camera system designed to capture images at a sampling density of 1024.times.768 pixels will typically contain a single image sensor array of 1024.times.768 sensors with a patterned overlay of red, green and blue color separation filters. From the single signal generated at each sensor location, three, 8-bit color signals for each of the 1024.times.768 sensor locations must be "reconstructed" or derived to provide a full-color image at the resolution of the sensor. This 1024.times.768.times.24-bit image is then stored in the camera (usually in compressed form) and subsequently transferred to a computer system for imaging on a display or for printing on a color printer.
FIG. 1 schematically illustrates an example of this most common configuration of color image capture system 20 in a color digital camera, a single-path sensor system. The image capture system 20 includes a lens 22 and a two-dimensional sensor array (e.g., charge coupled device array) with an integral Beyer color filter (red-green-green-blue) pattern 24 having a redundant green element 26. This particular color filter pattern is commonly used in charge coupled device-based color video cameras. The higher density of green-filtered sensors enhances the effectiveness of estimating the spatial luminance pattern. The class of methods for reconstructing the three, 8-bit color signals at each pixel location are known as "de-mosaicing operations" and are most commonly based upon linear interpolation and block replication procedures. Methods of feature estimation via templates have also been used.
FIG. 2 illustrates a second known approach to color separation for digital cameras, a three-path sensor system. The color separation system 30 includes prism optics and dichroic elements 32 to separate incoming light into three different broad spectral regions C.sub.1, C.sub.2 and C.sub.3, which are then imaged on three separate image sensor arrays, 34, 36 and 38, respectively. In principle, this approach preserves color image integrity and eliminates the need for de-mosaicing and its associated system overhead. This approach is typified by the Minolta RD-175 digital camera.
The most recent approach that has been proposed for color separation in image capture systems is based on temporal imaging (also known as field-sequential imaging) of red, green and blue image components onto a single sensor array. This approach has the potential cost and form factor advantages of single-path sensor systems and the more complete and efficient color separation performance of three-path sensor systems. While no known systems of this type currently exist in consumer level products, a field-sequential color separation element based upon fast-switching, nematic liquid crystal .pi. cells has been proposed.